1. Field of the Invention
The present invention relates to a camera and a camera system such as a single-lens reflex camera capable of TTL light measurement, and more particularly to a camera and a camera system having a depth of field preview mechanism for controlling the aperture diaphragm of an objective lens to a prescribed aperture value prior to photograghing operation.
2. Description of the Prior Art
There are known cameras having a depth of field preview mechanism (hereinafter call "preview mechanism") of the type described above.
However, in a camera which measures light transmitted through its objective lens in the condition its aperture diaphragm is fully opened and stores the measured data for use in exposure calculation, the following problem would ocurr by the provision of preview mechanism. If so-called AE lock is carried out in such a camera for storing the measured value while the aperture diaphragm is controlled by the preview mechanism to a prescribed aperture value, then the measured value on exposure measurement at a stopped down aperture remains stored even when the preview mechanism is thereafter released. Exposure calculation is therefore performed on the measured value that is stored, with the result that an error is produced which corresponds to the difference between the fully opened aperture value of the lens and the aperture value at the time the preview mechanism is in operation. Specifically, if a stored value Bv1 measured with the aperture stopped down by the preview mechanism should be used for exposure calculation as a measured value Bv0 with a fully opened aperture, then the difference Bv0-Bv1 would become an exposure calculation error.
Camera systems capable of changing lenses such as single-lens reflex camera systems are arranged such that exposure calculation is correct when a predetermined reference lens is mounted on the camera. When various lenses are changed on the camera, measured values of light for the same luminance in TTL light measurement at the fully opened aperture for those lenses vary from that of the reference lens.
There has already been proposed a camera system in which such a measurement difference is stored as an open-aperture measurement error Wvi in a ROM in each of the interchangeable lenses, and the stored data is used for correction in exposure measurement.
Where a preview mechanism capable of stopping down the aperture diaphragm to a prescribed aperture value is incorporated in such a camera system, since the wide-open-aperture measurement error Wvi is reduced when the preview mechanism is in operation, the above correction in exposure measurement will be excessive, and no proper exposure calculation will be effected. This problem will be described in more detail with reference to FIG. 1 of the accompanying drawings.
The curve B in FIG. 1 shows the relationship between an aperture value (F value) and the output Pa of a photometric cell, the relationship being linear with aperture. The curve D indicates the relationship between the aperture value (F value) and the output Pa of the photometic cell at the time an interchangeable lens having a minimum F value of 2.8 is mounted. Usually, the cell output Pa produced on exposure measurement at the fully open aperture when the interchangeable lens is mounted is different from that when the reference lens is mounted, by an open-aperture measurement error Wvi. Therefore, when an interchangeable lens is mounted, no accurate exposure control would be achieved unless the open-aperture measurement error Wvi were compensated for. Heretofore, the open-aperture measurement error Wvi has been compensated for by effecting an automatic exposure adjustment on exposure measurement with the fully open aperture. A result of such error compensation is indicated as E in FIG. 1.
The open-aperture measurement error Wvi is increased when the aperture is fully open, but is reduced as the aperture is stopped down by the preview mechanism. That is, as the aperture is stopped down, the outputs produced by the photometric cell at the time of changing lenses are rendered more uniform, and error compensation would be too excessive if effected during operation of the preview mechanism.
The output of the photometric cell which detects light coming from an object through the lens is not in linear relationship to the aperture value; it is especially nonlinear nearby the fully open aperture value. In TTL light measurement, it is known from the cos.sup.4 law and vignetting that the brightness of the edge of the image field is less than that of the center thereof. Such a reduction is greater as the aperture is closer to the fully open aperture value. Therefore, in the vicinity of the fully open aperture value, if the aperture is stopped down by equivalent 1 Ev, the output of the photometric cell does not vary equivalent to 1 Ev. Where this error is corrected so as to be acceptable at exposure measurement at the fully open aperture, an error is produced of necessity for exposure measurement at the stopped down aperture.
The above problem will also be described in greater detail with reference to FIG. 1. The curve A indicates the relationship between the aperture value (F value) and the output Pa of the photometric cell at the time the reference lens is mounted. If adjusted for automatic exposure control, the vertial axis of the FIG. 1 graph corresponds to a shutter speed (S.S.) that is automatically set. As described above, the curve B is the linearly adjusted relationship between the cell output Pa and the aperture value (F value) on exposure measurement at the open aperture, corresponding to the shutter speed when the reference lens is mounted. Comparison between the curves A and B indicates that they coincide with each other at the open aperture, but differ from each other when the aperture is stopped down. This means that when a certain aperture value, other than the open aperture, is set, the shutter speed which is automatically set by the result of exposure measurement at the open aperture is different from the shutter speed which is automatically set by the result of exposure measurement when the preview mechanism is operated.
In camera systems in which lenses are interchangeable such as single-lens reflex camera systems, the measured value of light which is obtained by TTL light measurement is stored in a memory, exposure calculation is effected on the basis of the stored value, and then an aperture value and a shutter speed are displayed. Where a preview mechanism is combined with such a camera system, when a power supply battery is detached while the preview mechanism is in operation, the data stored by the operation of the preview mechanism is lost, and so the correct aperture value will not be displayed when the power supply battery is attached again.
When the lens is detached while the preview mechanism is in operation, an aperture presetting lever on the camera body is in position corresponding to the aperture value set by the operation of the preview mechanism. If the minimum F value of a newly mounted lens is different from that of the detached lens, the aperture value displayed on the camera body remains to be that before the lens was changed, whereas the actual aperture value is a value that should be reduced from the open aperture value for the newly mounted lens through a number of steps determined by the aperture presetting lever. As a consequence, different aperture values are experienced before and after the lenses are changed. An error will therefore be caused if the user thinks that the newly mounted lens will be controlled by the aperture value that was displayed prior to the lens replacement.
Where a zoom lens of variable focal length is mounted on a camera system having a preview machanism, when zooming is effected while the aperture is stopped down by the preview mechanism to a diameter corresponding to a certain aperture value, the aperture value varies with such zooming. In an ordinary camera system, the aperture value immediately prior to the operation of the preview mechanism remains displayed, and hence the aperture value displayed during operation of the preview mechanism with a zoom lens is not an accurate aperture value.
If so-called AE lock for storing the measured value of light at the time of manual operation is effected in such a camera system while the preview mechanism is being operated, the measured value at the instant of manual operation is stored, and an appropriate shutter speed is calculated on the basis of the stored value. Where a zoom lens of variable focal length is employed in such a camera system, when zooming is carried out while AE lock is set during operation of the preview mechanism, the amount of light falling on a film varies since the aperture value changes with zooming, whereas the calculated shutter speed remains unchanged because of the AE lock. This results in an exposure error.
OBJECTS SUMMARY OF THE INVENTION
It is an object of the present invention to provide a camera capable of eliminating an error which would otherwise be produced by using a measured value Bv1 at a stopped down aperture while a preview mechanism is in operation and AE lock is set, as a measured value Bv0 at a fully open aperture.
Another object of the present invention is to provide a camera system capable of preventing excessive compensation of the above described open-aperture measurement error Wvi when a preview mechanism is operated.
Still another object of the present invention is to provide a camera system which causes no error in exposure display and exposure control during exposure measurement at a stopped down aperture through operation of a preview mechanism.
A still further object of the present invention is to provide a camera having an AE lock mechanism and a preview mechanism, the camera producing no exposure display and control error while the AE lock mechanism is in operation before the preview mechanism is operated.
Still another object of the present invention is to provide a camera system capable of giving the user a warning to prevent an exposure error from being produced due to erroneous display of an aperture value.
Still another object of the present invention is to provide a camera system capable of accurately displaying an aperture value even during operation of a preview mechanism.
A still further object of the present invention is to provide a camera system which prevents an exposure error from being produced even if zooming is performed while both a preview mechanism and an AE lock mechanism are in operation.
A yet further object of the present invention is to provide a camera having a preview mechanism and an AE lock mechanism, the camera allowing the preview mechanism to operate even during operation of the AE lock mechanism and storing an accurate measured value of light in such operation of the preview mechanism.
The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings.